The invention relates to a communication device having multiple antennas available to it for receiving a transmission.
Packet-based narrow-bandwidth radio systems, such as Bluetooth, are vulnerable to a variety of interference mechanisms which are collectively and colloquially grouped together as multipath and fading. In these interference mechanisms, the route or routes taken by radio signals from transmitting to receiving antenna cause either no signal to be received, or (due to destructive interference) a badly distorted signal to be received. The nature of these “fades” is such that they tend to be narrowband in nature, and in a frequency-hopping system such as Bluetooth it is likely that the hop onto the next channel will result in a clear fade-free channel.
Fading therefore causes occasional packet loss in a frequency-hopping packet-based system. When asynchronous data is being transferred across such a system—a file transfer, for example—and where mechanisms are available to detect such a loss and to cause a retransmission on another channel, then no significant impact (other than a minor loss of delivered link bandwidth) is incurred. When synchronous data, however, such as sample data representing voice traffic, is being transferred, then retransmission mechanisms are typically either not available or cause the link to use additional power or to have additional latency. Packets lost due to fading may therefore not be recovered and the resultant data loss may result in an audible distortion to the voice traffic.
A variety of proprietary techniques are available to perform Packet Loss Concealment, which involves the use of heuristic algorithms, typically implemented using digital signal processing techniques, in order to reconstruct a plausible version of the lost data. These post-hoc techniques tend to result in increased power usage and add latency.
It is therefore preferable to avoid packet loss.
Antenna diversity, which involves the use of two or more separated antennas, is a known technique for mitigating the effects of multipath/fading in RF systems. In systems operating within the ISM band, such as Bluetooth, the required separation between the antennas to render the technique useful is sufficiently small to make it of interest in applications within small devices, which can typically be either hand-held or worn about the body.
Current state-of-the-art for multi-antenna systems in ISM radios is for such systems to either use two or more receive chains or operate in a post-hoc fashion. In the first case, each antenna is connected to its own receive chain so that the expected packet is received by both of the separated antennas. The results are compared during or after reception and the “best” packet is chosen. In the second case, typically used when a single receive chain is implemented, an antenna is chosen for the upcoming reception based on historical data about the “quality” of reception of packets on the available antennas in the past. This latter case (multiple antennas, a single receive chain and post-hoc processing) brings with it the following unavoidable problems:
(1) Packets generally have to be lost in order to trigger a switch to the “other” antenna.
(2) After any switch has occurred, such a system has no knowledge of the quality of received signal at the “other” antenna without attempting a reception. In a case where the “current” antenna is receiving with marginal quality, making a switch could result in an improvement or in a lost packet, but there is no possibility of finding out which of these outcomes is the more likely without risking a lost packet.
(3) The response latency of such a system is very significant—often so large as to render it useless. A Bluetooth device transmitting SCO voice data and hopping across all 79 of its channels (a typical use-case) will (on average) re-use any given channel only every 290 ms or so. This means that information about fading conditions on a particular channel/antenna combination may well be out of date by the time that the channel is next used. This difference between rapidly-changing fading conditions (such as would be encountered, for example, by a headset-wearing user walking through a steel-framed building) and the response time of any antenna selection algorithm represents a major mismatch between such algorithms and the required response.
These problems mean that a degree of packet loss is unavoidable in a system using post-hoc antenna quality processing. In addition, the response time of such systems may render their application largely fruitless. The undoubted theoretical benefits of antenna diversity have not been realised in practice within mobile Bluetooth devices.
Therefore, there is a need for an improved technique for enabling a device to select between multiple antennas.